As to protect objects, such as for example documents and papers of value, but also spare parts, drugs, textiles or other trademarked products of the consumer goods sector from imitation and forgery as well as tampering, and for being able to test the authenticity of these objects, they are equipped with special security elements. Known security elements are, for example, watermarks or security threads incorporated into paper, luminescent or magnetic particles, which for example are added to printing inks, or embossed holograms, which are applied as a foil element to the surface of the objects or their packaging to be protected. For producing known security elements substances and pigments are used, which due to their particular optical properties cannot be imitated or only with great effort and which are not easily availably for everyone. This, for example, is the case for thin-film pigments, which when viewed or illuminated from different angles show different colours. Markings, such as prints, containing such pigments, for that reason are also referred to as “optically variable”.
The group of optically variable materials also includes liquid-crystalline materials, the optical properties of which, such as absorption and reflection, in an aligned state are dependent on direction. Due to their superior processing properties, for security elements in particular cross-linked liquid-crystalline polymers are possible, which, for example, can be present as a thin foil-like layer or in the form of pigments. As small plates or as a pigment these liquid-crystalline polymers can be added to printing inks and lacquers or plastics and other polymeric binders, and processed with conventional techniques, such as for example printing, lacquering or methods of foil technology. The production and processing of pigments consisting of liquid-crystalline material is described for example in EP 0 601 483 A1.
When viewing liquid-crystalline material with chiral phase, not only the perceived colour dependent on the viewing angle changes, but also the light reflected from this material is circularly polarized. Depending on the molecular structure of the material, the polarization can be left-handed circular or right-handed circular. Security elements, which contain respective liquid-crystalline materials, due to their optically variable properties, have the advantage that, for example, their direction-dependent optical properties cannot be reproduced or imitated even by high-quality copy machines, scanners or photographic devices.
For example from EP 0 435 029 A1 it is known that data carriers, such as papers of value and documents, are equipped with security elements, which contain a liquid-crystalline material. For their authenticity testing colour filters, quarter-wave plates, beam splitters and linear polarizers and a pair of detectors are used. By comparing the signal strengths present at the two detectors, conclusions about the authenticity of the security feature can be drawn.
WO 94/02329, too, describes the use of liquid-crystalline materials for the protection of documents and papers of value. These materials are applied to translucent areas, in particular above watermarks. An authenticity testing is made, for example, with the help of detectors, which detect the portions of light that are transmitted and reflected by the security element. For the automatic evaluation by machine in particular stripe patterns are proposed, the liquid-crystalline material in neighboring stripes differing in its handedness.